The phrase "is tough" can be substituted for a multitude of words, depending on the context of the sentence. For example, instead of saying "this problem is tough to solve," one could say "this problem is challenging" or "this problem is difficult." Other synonyms for "is tough" include "arduous," "demanding," "strenuous," "troublesome," "problematic," "hard," "frustrating," "complex," and "daunting." It's important to consider the intended meaning of the sentence and choose a synonym that accurately captures the intended tone and complexity of the topic.